This project serves to consolidate and to expand our efforts at identifying, localizing, and characterizing genes and gene products which are important in host pathogen relationships. Recent experiments have shown that nuclease-protected DNAs are exported from Borrelia burgdorferi cells in association with membrane vesicles. Indirect evidence suggests that these vesicles may be produced by spirochetes in vivo, providing a sustained antigenic challenge to hosts that maintain a limited population of spirochetes. To determine whether B. burgdorferi vesicles occur in experimentally infected mice, polyclonal rabbit sera were generated against vesicles and against a prominent, vesicleassociated protein and with an apparent mass of 83 kDa. Using these reagents, we developed an immune electron microscopic assay for first capturing and then identifying extracellular B. burgdorferi antigens. We then extended the study to include antigen detections in Ixodes ticks and in mouse, dog, and human samples. This approach has proven effective for demonstrating B. burgdorferi infections, and should enable further ultrastructural examination of the effects of these bioproducts on hosts.